In many battery-powered applications, including portable computer systems and telecommunications equipment, the battery of choice has been the Nickel metal hydride ("NiMH") variety. NiMH batteries produce relatively large amounts of constant voltage and constant current power per pound and are able to endure thousands of charge/discharge cycles during their lifetime. They are also portable and safe to use.
These and other rechargeable batteries do, however, have operating peculiarities. First, the performance of such batteries usually degrades over time. This degradation manifests itself more in the total amount of power the battery can deliver per charge cycle, rather than in a drop in voltage or current of the power. Second, many types of rechargeable batteries have a memory effect, whereby if the battery is used only partially and then recharged, the battery will, in effect, "remember" that it was only partially used. If this half-use/recharge cycle is repeated often enough (typically more than 20 times in a row), the battery will lose its ability to take a complete charge. To prevent this loss of capacity, the battery must undergo an occasional complete discharge/charge cycle (a so-called "reconditioning" or "deep discharge"). Third, the efficiency with which some of these batteries (e.g., a NiMH battery) recharge is a function of both the extent to which it has degraded over time and its memory.
In an effort to keep track of the amount of charge stored in a rechargeable battery, many battery-powered appliances are provided with battery charge level monitors and low power indicators that inform the user when the voltage level of the battery falls below a predetermined value. More elaborate charge level monitors track and indicate the charge level of the battery over a broad range of anticipated values. These features usually increase the overall cost of the appliance, sometimes significantly.
Such battery charge level monitors have been embodied in a programmable microcontroller associated with the battery. The programmable microcontroller has associated memory, often in the form of a register, employed to store charge data representing an estimated quantity of charge remaining in the battery. The microcontroller may derive the estimate from a preprogrammed static self-discharge table or algorithm. Such a microcontroller-battery combination is defined as a "smart battery." Portable computer systems commonly employ these smart batteries.
Another method for monitoring battery discharge within a portable computer system involves programming the microcontroller to act as a simple timer that is set immediately after the battery is charged. The value of the timer initially represents an estimate of the overall time the battery can power the system without recharging. As the computer system operates, the timer decrements at a given rate such that the user can determine how long the system can continue to operate.
Unfortunately, this method has many drawbacks. For instance, failure to set the timer or improper setting of the timer results in inaccurate predictions of the time of power loss. Similarly, if the battery has been only partially recharged, there is no reference point from which to estimate the time of power loss. Furthermore, this method does not take into account that some operations may consume more power than other operations.
More accurate methods have therefore been proposed in which the microcontroller varies a charge value to account for the amount of charge replenished by a battery recharger and the amount of charge consumed by specific operations. Although battery charge level monitors employing such methods have been quite successful in accurately estimating the power loss that results from the active operations of the computer system, such monitors do not track the amount of self-discharge that occurs within the battery when the system is turned off. In some situations and particularly when the computer system remains off for extended periods, self-discharge can account for a significant percentage of the overall charge depletion of the battery.
Unfortunately, these battery charge-level monitors do not take into account degradation of the rechargeable battery over time, as noted above. These monitors furthermore do not take an active role in reducing or eliminating any memory effects in rechargeable batteries, leaving the job of battery management, including reconditioning, to the user. Therefore, existing battery monitors and management apparatus do not provided users with overall, user independent battery state and condition information in an appropriately integrated fashion.
Accordingly, what is needed in the art is a way to accurately determine, in a cost-effective, integrated fashion, the condition of an in situ battery.